Circum-Pacific correlation of Argentine Early and Middle Jurassic bivalve zones

DAMBORENEA, S.E.

Poitiers, Francia

Simposio; 3rd International Symposium on Jurassic Stratigraphy; 1991

Detailed  knowledge of Jurassic bivalves has increased their use in biostratigraphy. Although bivalve biostratigraphic units have local or regional rather than wider application, a correlation of such units recognized in different regions around the Pacific is attempted. The same bivalve groups and frequently the same genera are used to establish biostratigraphic units in both northern and southern circum-Pacific regions- These involve monotaceans and pectinaceans for the Early Jurassic and inoceramids for the Middle Jurassic. Five Early Jurassic Assemblage Zones have been recognized in Argentina: Otapiria pacifica, O. neuquensis, Radulonectites sosneadoensis, Posidonotis cancellata and Propeamussium pumilum. Similarly, one faunule and four Zones are proposed for the Middle Jurassic of Argentina on the basis of the distribution of inoceramid faunas: Retroceramus cf. inconditus faunule, Parainoceramus? westermanni, R. cf. marwicki, R. patagonicus and R. stehni Zones. All of them are accurately calibrated against the local ammonite zonation. Each one is here correlated with biostratigraphic units proposed for other circum-Pacific regions on the basis of bivalves. Relations with Northeast and Far-East USSR detailed bivalve zonation are particularly interesting because they show approximately equivalent distributions in time for similar groups despite the distances involved. A slight displacement in time appears between similar faunas from both regions: most South American assemblages consistently are somewhat younger than their Boreal counterparts. These shift may reflect progressive migration from North to South, but other causes should also be considered. The comparison shows that direct correlation between bivalve zones is possible, especially between relatively close regions. Under this light, knowledge outlined from Andean faunas can be applied to adjust the age of local stages recognized in New Zealand and other South Pacific areas, which are mostly based on bivalves.